HTML is a free form space - you can put material most anywhere on the screen. Excel is built on a row and column grid, so the export mechanism has to try and match the column structure of the spreadsheet to the positions of the columns in your report. If you have multiple tables in your report, you will get a spreadsheet with some large numbers of small columns with merged cells.

When planning a report that will be exported to Excel, you need to think about Excel's limitations first. Minimize the number of tables. Conditionally suppress elements that willl not be exported so that the Excel emitter does not try and allow for them in the output computations.

You really need to find someone with BIRT knowledge who speaks one of your native languages.

The BIRT Excel emitter will automatically generate merged cells when the report has multiple elements with overlapping borders. That's not something you have control over.

For example, if I have a top grid, three columns, widths 1 inch, 5 inches, 1 inch;
then I have a data table, five columns, widths 1 inch, 2 inches, 1 inch, 1 inch, 2 inches;

The output exported to Excel will have seven columns; the top grid will have cells in column A, columns BCDEF merged, column G. Then after a blank row, the data table will have cells in column A, columns BC merged, column D, column E and finally columns FG merged.

That is a vast over simplification (and ignores the effects of cell margins and padding) but it should give you an answer to your question.